1. Field of the Invention
The present invention relates to a negative pressure booster used for a toggle operation of a brake master cylinder of a motor vehicle. More particularly, the invention relates to an improvement for a negative pressure booster including: a booster shell; a booster piston which is accommodated inside the booster shell and partitions the interior thereof into a front negative pressure chamber communicating with a negative pressure source, and a rear operation chamber; a valve cylinder communicating with the booster piston; a valve piston fitted into the valve cylinder to be slidable in a forward and backward direction of the valve piston; an input rod coupling with the valve piston at a front end thereof; a control valve for switching to communicate the operation chamber with either the negative pressure chamber or an air between the valve piston and the valve cylinder, in accordance with the forward and backward movement of the input rod; an output rod slidably fitted into the valve cylinder; and a repulsion force mechanism provided among the valve cylinder, the valve piston and the output rod, and including a flexible piston that is positioned between the valve cylinder and the output rod, a repulsion force piston that is positioned facing the flexible piston on the side opposite the output rod and that has a smaller diameter than the flexible piston, and a spring that is positioned between the repulsion force piston and the valve piston and that imposes a setup load thereon in the direction of contraction.
2. Description of the Related Art
Such a negative pressure booster is already known as disclosed in, for example, Japanese Patent Unexamined Publication No. Hei.10-100887.
In a conventional negative pressure booster, a flange is formed on the outer periphery of the middle portion of the repulsion force piston, and a retainer is coupled with the rear end of the repulsion force piston so as to be slidable in a constant stroke. Between the retainer and the flange, a spring is so arranged that when the spring has been contracted by a predetermined amount, the retainer abuts upon the repulsion force piston, and an input obtained from the input rod is transmitted from the valve piston, through the retainer, to the repulsion force piston and the flexible piston. Thereafter, the output reaches a toggle limit point.
As shown in the output characteristic graph in FIG. 5, in such a conventional negative pressure booster the toggle force ratio, i.e., the ratio of the output to the input, is increased in a region where the spring is contractedly deformed, and is reduced after that region has been passed. Thus, the output characteristic is shifted away from a secondary curve ideal for braking, so that deterioration of the operating function occurs.
To resolve this problem, it is one objective of the invention to provide a negative pressure booster for approximating the output characteristic of an output rod to a secondary curve ideal for braking until the output reaches the toggle limit point.
To achieve the above objective, according to a first aspect of the invention, a negative pressure booster comprises: a booster shell; a booster piston accommodated inside the booster shell and partitioning the interior thereof into a front negative pressure chamber communicating with a negative pressure source, and a rear operation chamber; a valve cylinder communicating with the booster piston; a valve piston fitted into the valve cylinder to be slidable in a forward and rearward direction of the valve cylinder; an input rod coupling with the valve piston at a front end thereof; a control valve switching communication of the operation chamber with the negative pressure chamber and with air in accordance with a forward and rearward movement of the input rod; an output rod slidably fitted into the valve cylinder; a repulsion force mechanism provided among the valve cylinder, the valve piston and the output rod, the repulsion force mechanism including: a flexible piston interposed between the valve cylinder and the output rod; a repulsion force piston facing the flexible piston on the side opposite the output rod and having a small diameter than the flexible piston; and a spring interposed between the repulsion force piston and the valve piston and imposing a setup load thereon in the contraction direction; and a stopper provided in the valve cylinder and supporting a front end of the valve piston before the spring is reached at the contraction limit.
According to this aspect, when the input provided via the input rod is less than the setup load imposed on the spring while the input rod is moved forward, the output produced by the output rod is increased at a toggle ratio consonant with the ratio of the pressure imposed by the repulsion force piston on a reception area to the pressure imposed by the operating piston that abuts upon the flexible piston, because the input rod and the repulsion piston form a single rigid member.
When the input provided via the input rod exceeds the setup load imposed on the spring, the spring is compressed between the valve piston and the retainer, and one part of the repulsion force that is exerted by the flexible piston on the repulsion force piston is absorbed, so that the toggle ratio seems to be increased.
Since the toggle limit point is reached by bringing the valve piston into contact with the stopper of the valve cylinder before the limit of the contraction of the spring is reached, the reduction of the toggle ratio can be avoided. Thus, for effective braking, at the toggle limit point the output characteristic of the output rod can approximate an ideal secondary curve.
Further, in addition to the first aspect, according to a second aspect of the invention a support shaft projects outward from the front portion of the valve piston, and the spring is positioned between the valve piston and a retainer, which is coupled with the support shaft, so that it slides along the support shaft at a constant stroke distance.
According to this aspect, a coil spring and the retainer, which are small parts, are coupled together with the valve piston to constitute an assembly that communicates with the input rod. Thus, during the assembly of the negative pressure booster, the small coil spring and the retainer can be attached to the valve cylinder, at predetermined locations, at the same time as the input rod is inserted into the valve cylinder. As a result, an efficient assembly process can be implemented.